Transparent touch-sensitive switching system

ABSTRACT

The invention pertains to an at least partially transparent touch-sensitive switching system comprising at least two electrodes provided with means for applying a voltage thereto and spaced from each other by a layer comprising at least one region that optically changes by applying the voltage, and at least one region comprising a piezoelectric material generating a voltage when applying pressure thereto.

The invention pertains to an at least partially transparenttouch-sensitive switching system and an electro-optical displaycomprising said touch-sensitive switching system.

A transparent touch-sensitive switching system was disclosed in U.S.Pat. No. 4,516,112. According to this patent a display was made with adisplay screen. Above the display screen a piezoelectric film wassupported for selective localized depression towards the screen. Thefilm has transparent flexible conductive coatings on opposite sides ofthe piezoelectric film. These transparent flexible conductive coatingsare necessary to generate a voltage in response to depression of thefilm. This system has the disadvantage that production thereof isexpensive, since the piezoelectric film must be provided with two extratransparent flexible conductive coatings. Moreover, the addition ofthese transparent flexible conductive coatings has a disadvantageouseffect on the transparency of the display, particularly in displays thatoperate with low power consumption, such as displays that are in use inmobile systems like mobile telephone displays, notebook computersystems, and the like. Further, these displays are thick and thereforerelatively heavy, providing an other disadvantage, particularly inmobile systems.

There is thus a serious need for improvement of the aforementionedsystem, in respect to manufacturing costs, optical performance (i.e.transparency), weight, and low energy consumption.

The present invention provides an improved at least partiallytransparent touch-sensitive switching system comprising at least twoelectrodes provided with means for applying a voltage thereto and spacedfrom each other by a layer comprising at least one region that opticallychanges by applying the voltage, and at least one region comprising apiezoelectric material generating a voltage when applying pressurethereto.

The present system can be manufactured at relatively low costs and hasexcellent optical properties. It should be stressed that thepiezoelectric material is preferably a piezoelectric polymer. Suchpiezoelectric polymer can replace the conventional polymer that ispresent in almost any electro-optical display. Thus the displays of theinvention can be made at about the same costs as conventional displaysby simply using a piezoelectric polymer matrix material rather than anormal polymeric matrix material.

Other transparent touch-sensitive switching systems are known, such asdisclosed in U.S. Pat. No. 5,159,323 wherein a display is disclosed witha piezoelectric material disposed between two substrates. However, thissystem is incomparable with the present system in that it makes use of aunique ferroelectric liquid crystalline material that inherently alsohas piezoelectric properties. Thus this material is used both as lightinfluencing medium and as a pressure sensitive medium. Such system isvery restricted in materials than can be used, and consequently veryrestricted in their applications. In contrast, the present inventionprovides in a system of a touch-sensitive switching method that can becombined with any material that can give an optical (light) effect,including non-liquid crystalline materials.

The present system allows the manufacture of display cells, which upontouching generates a voltage in the piezoelectric material that servesto detect the touch position. Examples of displays wherein thetouch-sensitive system of the invention can be used are PDLC (polymerdispersed liquid crystal), electrophoretic displays, Gyricon™, PDCTLC(polymer dispersed cholesteric texture liquid crystal), polymerdispersed guest/host systems, and other polymer dispersed LC effects,systems with pixel walls (electrochromic, electrowetting, ASM (axialsymmetric microcell) mode LCD, and the like). Preferably, thepiezoelectric material is a polymer or a polymer comprisingpiezoelectric particles, which may be applied to make special spacerconstructions, such as in a lithographic manner as disclosed by Odaharaet al. in SID 01 Digest, p. 1358, or by micro-embossing. It is however,more preferred to incorporate the piezoelectric material in the pixel.In another preferred embodiment a display is made wherein at least oneof the conductive or semi-conductive layers is placed onto a substrate.The substrate may be any material that is commonly used for thispurpose, such as glass or plastic, among which polycarbonate,polyurethane and the like. In a preferred embodiment according to theinvention the touch-sensitive switching system has at least one of theelectrodes placed onto a substrate. The touch-sensitive switching systemis at least partially transparent. For many applications it is preferredthat the system is fully transparent.

The piezoelectric materials are known in the art and commerciallyavailable. Most commonly used is polyvinylidene fluoride (PVDF) film,for instance having a thickness of 10 to 100 μm, but it is clear thatany other piezoelectric material can also suitably be used, such as aferroelectric (chiral smectic C*) elastomer (Brahma, M., Wiesemann, A.,Zentel, R., Siemensmeyer, K., Wagenblast, G., Polymer Preprints, 1993,34 (2), 708;bttp://staudinger.chernie.uni-mainz.de/akzentel/Publikationen/p991.htm#13), and polymers comprising piezoelectric particles (e.g. atitanate).

When using a piezoelectric polymer, the molecular alignment by applyinga high-poling voltage creates the piezoelectric effect. Thus therandomly oriented piezoelectric material is ordered on applying ahigh-voltage poling voltage. Touch signals may then be measured over thepixel electrodes that are already present to drive the pixels, thus noextra connections are necessary in the display. By identification whichrow and column has generated a voltage upon applying pressure, sensingcan be carried out.

The invention is further illustrated by the following figures.

FIG. 1 shows schematically a touch-sensitive pixel according to theinvention with a piezoelectric polymer and regions containing liquidcrystalline material.

FIG. 2 shows schematically a touch-sensitive pixel according to theinvention with a piezoelectric polymer and capsules containingelectrostatically charged particles.

In FIG. 1 a first substrate 1 is provided with an ITO layer (conductivelayer) 2. A second substrate 3 is also provided an ITO layer 4. The twosubstrates are spaced apart by a polymer layer 5, having piezoelectricproperties, by being piezoelectric as such or by comprisingpiezoelectric material. In polymer layer 5 regions 6 are present whichmay be “floating” droplets comprising liquid crystalline (LC) moleculesor may be a capsule filled with LC molecules. If a capsule is used suchcapsule is made of a thin transparent polymeric film that are commonlyused for making capsules. The regions that optically change by applyingthe voltage are embedded in a matrix of the piezoelectric polymer. Means7 are present for applying a voltage over the ITO layers 2 and 4.

The layer 5 can be exposed to a high electric field in order to alignthe piezoelectric polymer.

In this way, the entire pixel region is touch-sensitized, which makesthis touch-sensitive approach applicable for flexible and even wearabledisplay applications.

According to this embodiment the touch signal over the pixel electrodesis measured by the connections already present to drive the pixels.Sensing is carried out by simply identifying the row and column thathave generated a voltage as a result of the touch pressure.

There method can be used for a wide range of liquid crystal systems thatrely on a polymer network to create the desired electro-opticalproperties. Examples are PDLC, PDCTLC, guest-host systems with polymernetworks (as used in camera shutter systems), etc. Each pixel maycontain a plurality of regions 6, but more preferably each region 6 isan individual pixel.

An embodiment of an LCD with polymer network where pixels areindividually encapsulated using a polymer network is the so-calledAxially Symmetric Mode (ASM) used for wide viewing angle LCD's and PALC(plasma addressed liquid crystal) displays.

In FIG. 2 a glass substrate 1 is provided with an ITO layer (conductivelayer) 2. A second substrate 3 is a PET layer with a thickness of175-250 μm, which is also provided with an ITO layer 4. The twosubstrates are spaced apart by a polymer layer 5, having piezoelectricproperties, by being piezoelectric as such or by comprisingpiezoelectric material. In polymer layer 5 capsules 6 are present filledwith electrostatically charged particles, wherein the particles aredispersed in a fluid (gas or liquid). Means 7 are present for applying avoltage over the ITO layers 2 and 4. Again each pixel may contain aplurality of capsules, but more preferably each capsule represents anindividual pixel. In this figure is indicated that the electrostaticallycharged particles are two different sorts of particles, whereof thecolors are in contrast to each other and which are dispersed in acolorless fluid. In this particular case the capsules contain black andwhite particles in a colorless fluid, for instance negativeelectrostatically charged carbon black particles and positiveelectrostatically charged white TiO₂ particles. Alternatively, theparticles can also be electrostatically charged particles having onlyone color that is in contrast to the color of the fluid.

In this case, the piezoelectric polymer will be used as binder. Theprinciple of the present invention may advantageously also be used inmany other types of electrophoretic display concepts that have beenproposed in patent applications. These concepts will also functionbetter if pixels are individually encapsulated using a polymer network.Again, this polymer network could be made piezoelectric to enable touchsensing. For instance, the known display principle called Gyricon™ (exXerox) could be made intrinsically touch-sensitive. Here, the polymernetwork forming the binder around the rotating balls with black andwhite hemispheres could be made piezoelectric to enable touch sensing.

In another embodiment according to the invention (not shown) twoelectrodes are placed on the same substrate and spaced from each otherby a layer 5 containing regions 6, according to layer 5 and regions 6 ofFIGS. 1 or 2. These electrodes may alternatively also be contained inthe same layer 5. The system of the invention may further contain one ormore of the usual other layers, such as substrate layers, intermediatelayers, compensation or retardation layers, polarization layers,protective layers, and the like.

Several concepts for electrochromic and electrowetting display pixelswill function better if pixels are individually encapsulated using apolymer network. Here also, this polymer network could be madepiezoelectric to enable touch sensing.

1. An at least partially transparent touch-sensitive switching systemcomprising at least two electrodes provided with means for applying avoltage thereto and spaced from each other by a layer comprising atleast one region that optically changes by applying the voltage, and atleast one region comprising a piezoelectric material generating avoltage when applying pressure thereto.
 2. The touch-sensitive switchingsystem of claim 1 wherein at least one of the electrodes is transparent.3. The touch-sensitive switching system of claim 1 wherein thepiezoelectric material is a piezoelectric polymer.
 4. Thetouch-sensitive switching system of claim 1 wherein the regions thatoptically change by applying the voltage are regions comprising a fluidor a dispersion of particles in a fluid.
 5. The touch-sensitiveswitching system of claim 4 wherein the fluid comprises liquidcrystalline molecules.
 6. The touch-sensitive switching system of claim4 wherein the particles are electrostatically charged.
 7. Thetouch-sensitive switching system of claim 6 wherein theelectrostatically charged particles have a color that is in contrast tothe color of the fluid, or wherein the particles are dispersed in acolorless fluid and the dispersion comprises at least two differentsorts of electrostatically charged particles, whereof the colors are incontrast to each other.
 8. The touch-sensitive switching system of claim1 wherein the regions that optically change by applying the voltage areembedded in a matrix of the piezoelectric material.
 9. Thetouch-sensitive switching system of claim 6 wherein the dispersion ofelectrostatically charged particles in a fluid is enclosed in a capsuleof a polymeric material.
 10. An electro-optical display comprising thetouch-sensitive switching system of claim
 1. 11. The display of claim 10wherein each region that optically changes by applying the voltagecorresponds to one pixel, and wherein each pixel is a pressure-sensitivepixel.